1. Field of the Invention
In general, the present invention relates to environmental control units for aircraft that have pressurized cabins. More particularly, the present invention relates to environmental control units that utilize secondary turbines to compress air, wherein the secondary turbines are driven by the bleed air from an aircraft engine.
2. Prior Art Description
Low flying, relatively slow aircraft do not have to have sophisticated environmental controls for the inside of the aircraft cabin. The quality of the air within the aircraft cabin can be adjusted by simply opening and closing vents or windows. However, many modern aircraft are designed to fly at high altitudes and at high speeds. Such aircraft require pressurized cabins, where the pressure within the aircraft is artificially maintained. If an aircraft cabin is pressurized, fresh ambient air cannot simply be vented into the pressurized cabin from outside the aircraft. Rather fresh air must be compressed to a pressure that matches that of the interior of the pressurized cabin so that the fresh air will flow into the pressurized cabin.
Aircraft that are designed to fly at high altitudes typically have jet engines or turboprop engines. Such turbine engines have compressors that can compress air to pressures above one hundred pounds per square inch. As the air is compressed, it is heated and may achieve temperatures over five hundred degrees Fahrenheit at sea level. Fuel is then added to the compressed air and ignited in a separate combustion section of the aircraft engine.
Air can be bled from the compression chamber of the engine, prior to the compressed air becoming mixed with the fuel. By bleeding some air from the engine, a source of high temperature/high pressure air can be obtained. In early designs for aircraft environmental control systems, engine bleed air was directly used to feed air into a pressurized cabin. Such an environmental control system is exemplified by U.S. Pat. No. 3,537,510 to Rennenberg, entitled Pressure And Temperature Regulator For Aircraft Bleed Air System.
Since, engine bleed air is typically at a high pressure and at a high temperature, sophisticated heat exchangers and pressure regulators must be used to condition the bleed air so it is at the right temperature and pressure to be introduced into the cabin. Should a component fail, high temperature bleed air will directly flow into the pressurized cabin, thereby quickly overheating the cabin and requiring the aircraft to land for safety concerns and repairs.
Although engine bleed air can be used to directly heat the passenger cabin of an aircraft, the bleed air itself is never cooler than the cabin and cannot be used to cool the cabin. Rather, to cool the passenger cabin of an aircraft, the hot, high pressure bleed air is used to turn an expansion turbine. The turbine drives a compressor that is used to blow ambient air through a heat exchanger, thereby producing cooling. An additional function of the expansion turbine in such prior art systems is to remove energy from the bleed flow air after it has been cooled by a heat exchanger. This cools the bleed air flow to a temperature below that of the aircraft cabin. The cooled bleed air can then be used to cool the aircraft cabin. Such prior art systems, however, are complex and require sophisticated heat exchangers. These systems also use substantial volumes of bleed air from the aircraft engines. Furthermore, in many modern environmental control systems, it is the expansion turbine that has proven to be the least reliable component of the environmental control system and the most likely to fail.
In a variation of such a prior art cooling system, a turbine driven by engine bleed air can itself be used to turn a refrigerant compressor. Thus, the turbine can be used to directly power a refrigeration system. Such prior art air conditioning systems are exemplified by U.S. Pat. No. 6,684,660 to Bruno, entitled Pneumatic Cabin Super Charger. However, such prior art systems still suffer from the unreliability and short operational life of the turbines.
One problem associated with such prior art environmental control systems is that although the engine bleed air is used to run an air cooling system, the bleed air is also directly used to provide heat and cabin pressurization. Thus, if a component fails in the environmental control system, engine bleed air is directly fed into the passenger cabin and the passenger cabin quickly overheats.
Another problem associated with such prior art systems is that a large flow of engine bleed air is needed to drive the cooling system. Thus, the cooling system may work well when the aircraft is in flight and the engine is at cruise power. However, when the aircraft is taxiing on the ground and the engines are at idle, the cooling system works poorly.
Yet another problem associated with many prior art environmental control systems is that they require large volumes of bleed air from the engines. This results in direct power losses from the engines since it starves the engines of the high pressure air needed during combustion. Excess removal of bleed air from an engine also results in higher engine operating temperatures and increased maintenance requirements.
A need therefore exists for an improved environmental control system for an aircraft that can provide both heated air and cooled air into a pressurized cabin without ever directing engine bleed air into the cabin. In this manner, should the environmental control system ever fail, engine bleed air will not flow into the pressurized cabin and the pressurized cabin will not overheat.
A need also exists for an improved environmental control system that can work efficiently to cool the cabin when the aircraft is on the ground and the engines are at idle.
A need also exists for an environmental control system that can introduce more air into an aircraft cabin than is drained from the engines, thereby increasing engine efficiency and power when the environmental control system is in use.
These needs are met by the present invention as described and claimed below.